This invention relates generally to electronic type platform weighing systems, and more particularly to an improved load cell and sensor plate for use in an electronic weighing scale.
There are many different types of electronic weighing scales in use today. One popular type of electronic weighing scale is constructed with a platform for receiving the load to be weighed and a set of levers, pivots, flexures, and torque tubes to mechanically combine the forces applied to the platform by the load, thereby enabling the measurement of these forces with a single electronic load cell. The load cell is typically constructed with a mechanically-deformable sensor plate which operates as a force transducer. The sensor plate has bonded thereto one or more sensor elements that serve to convert the mechanical bending forces of the sensor plate into electrical signals. When a load is applied to such a load cell, the sensor elements bonded to the sensor plate produce electrical signals which are proportional to the load applied to the load cell.
Another popular type of electronic weighing scale is constructed with a platform which is supported by several electronic load cells which each produce an output signal. The output signals of the load cells are electrically averaged to enable forces to be measured over a large area of the platform.
The sensor plates employed in the load cells described above, are of many different designs. One such load plate design is exemplified in U.S. Pat. No. 4,020,686 issued to Brendel.
The sensor plate disclosed therein is constructed with a rigid force input section, a rigid force output section, and an intermediate force moment carrying section coupled between the force input and output sections. The sections are interconnected by four horizontal flexural elements, four vertical flexural elements, a pair of force measuring members, and a pair of connecting structures. This design allows forces to be measured along a first axis of the sensor plate and also moments about a second axis of the sensor plate which is perpendicular to the first axis. However, the rather tall profile of this sensor plate design undesirably imposes minimum thickness requirements on the weighing scales.
Another load plate design can be seen in U.S. Pat. No. 4,993,506 issued to Angel. The sensor plate disclosed therein is fabricated from a single, flat metal stamping consisting of a flat flexure beam where strain sensors are bonded, a flat U-shaped loaded element attached to one end of the flexure beam, and a flat mounting element attached to the other end of the flexure beam.
Still another load plate design is described in U.S. Pat. No. 4,548,086 issued to Kastel. The sensor plate of Kastel is fabricated from an areal spring material and is provided with a force introducing section, a clamping section, and an intermediate section connected to the force introducing section and the clamping section by flexural webs which are equipped with sensor elements.
The sensor plates described in U.S. Pat. Nos. 4,993,506 and 4,548,086 are relatively thin and thus, are used in low profile scale designs which are presently very popular. Although these sensor plate designs are relatively inexpensive to produce compared with earlier designs, further reductions in machining costs and the like are desirable.
Furthermore, prior art systems also utilize sensor plates having surfaces which have corregated or slotted rather than flat planar surfaces. This causes significant problems in attachment of electronic wires and/or electronic devices to the plate. Still further, numerous problems exist in the prior art regarding off-axis application of a load to the sensor plate. Such misalignment causes calibration and/or weight calculation errors, thus minimizing the accuracy of the scale.
Accordingly, there remains a need for an improved sensor plate and a load cell for use in electronic weighing scales which is reliable, accurate, and substantially simple and economical to manufacture.
It is an object of the present invention to provide a sensor plate for use in a load cell for use in an electronic scale comprising a planar first surface; a planar second surface opposite the first surface having a depression formed therein defining a flexure area; a load cavity formed in the second surface having a conical receptacle end for receiving a strut; and sensor means disposed over the flexure area for generating a signal is response to a load applied to the loading cavity wherein the strut has a first projection end coupled to the conical end of the loading cavity and a second end coupled to a footer member such that the strut mechanically floats therebetween for providing the applied load at a substantially central position at the load cavity.